(1) Field of the Invention
The present invention relates to a process for the production of machinable vitreous ceramics suitable for machine parts and the like.
(2) Description of the Prior Art
Crystallized vitreous ceramics are often used as one of ceramic materials for mechanical or electrical parts. As the vitreous ceramics of this type, there is known a vitreous ceramic article in which at least 50% by volume of a crystal of a fluormica is dispersed, as disclosed in Japanese Patent Publication No. 34775/79. This vitreous ceramic is prepared by melting the starting composition to effect vitrification and crystallizing a fluormica crystal and contains fine crystals randomly, and the vitreous ceramics is nonpermeable and is easily machined by cutting or the like. However, the vitreous ceramics is too low in mechanical strength and is defective in that cracks are formed during the step of forming a vitreous body and the yield is therefore reduced.
Furthermore, Japanese Patent Application Laid-Open Specification No. 72654/86 discloses a process in which a fluorine-containing mixture comprising pottery stone as the main material in melted and the obtained vitreous body and an easily gasifiable fluorine compound are heated at 1100.degree. to 1360.degree. C. in a closed vessel to crystallize fluormica in the vitreous formed body. According to this process, crystallized vitreous ceramics can be obtained in a short time, but formation of cracks during the step of forming a vitreous body is not substantially eliminated and the strength is not substantially improved.
The above-mentioned conventional fluormica-containing vitreous ceramics can be machined by cutting or the like and have an appropriate mechanical strength, and when a small product is prepared by using these ceramics, no particular problem arises but when it is intended to prepare a large product (having a size of 15 cm.times.15 cm.times.3 cm or a larger size), cracks are often formed in the vitrified formed body (hereinafter referred to as "vitreous body") and the yield is reduced. Furthermore, it is desirable that the strength be improved. However, if the starting material composition and the preparation conditions are so selected that the strength is improved, the machinability is reduced.